In the field of medical technology, there are many known means by which liquids can be removed from patients, or liquids can be supplied to the patients, via a flexible tubing line. Access to the patient is generally obtained in this case via catheters for introduction into organs of the body or via needles or cannulas for insertion into vessels.
In methods for chronic blood-cleansing therapy such as hemodialysis, hemofiltration and hemodiafiltration, a patient's blood is passed through an extracorporeal blood circuit, which comprises an arterial segment leading to a blood-cleansing component in the form of a dialyser and a venous segment exiting from the dialyser. In the extracorporeal treatment of blood, there is a risk of air, which enters the blood circuit as a result of a leak, causing a life-threatening embolism in the patient. Air detectors, which are generally arranged in the venous segment of the blood circuit, are therefore used to monitor the extracorporeal blood circuit.
Methods of dialysis are known in which access to the patient is via a common needle which is connected to both the arterial and the venous segments of the blood circuit, or via two needles which are connected to the arterial segment and the venous segment, respectively, of the blood circuit.
In the single-needle method of dialysis, if there is a leak in the region of the needle, air may be drawn into the tubing during the arterial phase. Some of this air may be left in the tubing system in the region of the Y-connection between the arterial and venous segments, in which case there is a risk of some of this indrawn air being supplied straight to the patient in the venous phase without directly actuating the air detector in the venous segment. 20 ml of air, for example, may be drawn in during just one phase.
Depending on the cardiac output selected and the size of the dialyser, many phases may elapse before the air which is drawn in during the arterial phase is detected by the air detector in the venous segment of the blood circuit, and it is therefore possible that there may be a considerable delay in an error message being given.
The known monitoring systems for detecting air in the extracorporeal blood circuit sense changes in the optical, electrical or acoustic properties of the blood flowing in the extracorporeal blood circuit. If given limiting values are exceeded, an alarm is activated and any further circulation of the blood is stopped.
As well as the monitoring systems for detecting air, monitoring systems for detecting disruptions to the extracorporeal blood flow, such as stenoses for example, are also known.
DE 103 55 042 B3 describes a method of detecting disruptions to the blood flow in which the phase angle of at least one upper harmonic of an oscillating pressure signal propagating in the extracorporeal blood circuit is determined, disruptions to the blood flow being detected on the basis of a characteristic change in the phase angle of at least one upper harmonic of the pressure signal. A precondition set by the known method is that the blood pump must generate an oscillating pressure signal in the blood circuit.
Known from US Publication No. 2002/0174721 A1 is a method of detecting stenoses in a flexible tubing system during extracorporeal treatment of blood. To allow a stenosis to be detected, the frequency spectrum of an oscillating pressure signal, which is attributable to the operation of the blood pump and which propagates in the extracorporeal blood circuit, is analyzed. It is concluded that a stenosis exists if the attenuation of at least one upper harmonic of the oscillating pressure signal changes.
DE 100 33 192 A1 describes a method of detecting arterial input problems during extracorporeal blood treatment, in which the amplitude of cyclic variations in pressure in the venous blood line is measured and compared with a limiting value. It is concluded that there arterial input problems exist if the limiting value is exceeded. A precondition set by the known method is that the blood pump must produce cyclic variations in pressure in the blood circuit.